The Assassin's Creed franchise is often known for its historical accuracy. When it doesn't have you fighting Greek myths or taking astral trips into Asgard, that is. Yes, Assassin's Creed Valhalla and Odyssey before it took some liberties with the more magical elements of their settings, but the older games tended to reflect a desire to capture the era more closely. Take, for instance, Assassin's Creed Unity's globe here.
Unity takes place during the French Revolution of the late 1700s. During this time, the United States of America was a fledgling nation that had just emerged from the 13 original colonies. It would be a long time before California became a state, or was even properly charted. Instead, maps of the time showed the land as an island.
This came from maps of the 1600s, which worked from info given by early explorers as well as European legends told of the far-off land. In reality, the stories were talking about the Baja Peninsula. This landmass juts out from the California-Mexico border, separating a lot of mainland Mexico from the Pacific Ocean.
Because this long strip of land had water on three sides, the misconception of it being an island grew large enough to have it be on maps for years to come. Apparently even the Assassins lacked proper intel on California, as their secret base has this globe bearing the "Island of California."